The use of a variety of structured surface films in backlit displays is well known. For example, U.S. Pat. No. 5,161,041 (xe2x80x9cAbileahxe2x80x9d) describes a prismatic film to enhance the apparent brightness of a backlit liquid crystal display. The Abileah patent further describes the use of two such structured surface films, preferably with the structures oriented perpendicular to one another, to further enhance the apparent brightness of such a display. In general, the increase in on-axis brightness produced by such a structured surface film is known as the xe2x80x9cgainxe2x80x9d of such a film. The on-axis gain of a film refers to the ratio of the intensity of light as measured in a direction perpendicular to the backlight with the film to the intensity observed in a direction perpendicular to the same backlight without the film.
Other structured surface films may be used to redirect light traveling in one direction to a more desired output angle. U.S. Pat. No. 4,984,144 (xe2x80x9cCobb et al.xe2x80x9d) and U.S. Pat. No. 5,190,370 (xe2x80x9cMiller et al.xe2x80x9d), the teachings of which are incorporated herein by reference, teach light fixtures utilizing such films. These light fixtures could be used as backlights for displays.
One problem with using films such as those described above in a display that is intended for close viewing, such as a computer display, is that the cosmetic requirements are very high. This is because, when such displays are studied very closely or used for an extended period of time, even very small defects may be visible and annoying. Elimination of such defects can be very costly both in inspection time and discarded materials.
There are several approaches to the problems created by the visibility of such small defects and consequent low manufacturing yield. One solution is to simply accept the relatively high rejection rate of common manufacturing processes. A second solution is to provide more efficient, and expensive, clean rooms, use only ultra-clean materials, use extraordinary care in the preparation of manufacturing tooling, and employ extremely rigid quality control procedures. While this will reduce the waste, it can introduce even more expense in order to produce higher yield.
Another solution is to provide the film with a diffuser. This may be a matte finish on the smooth side, the structured side, or both, of the film or a bulk diffuser provided in the film. Such diffusers will hide many of the defects, making them invisible to the user. This will significantly improve manufacturing yield, while only adding a small increase in cost to the manufactured part. The disadvantage of this approach is that the diffuser will scatter the light and thus decrease on-axis gain. Therefore, a diffuser will increase yield but at the expense of some performance.
Another problem that has been observed in prior art systems utilizing two sheets of brightness enhancing film, as described above, is known as xe2x80x9cwet-out.xe2x80x9d Wet-out occurs as a result of optical coupling between the prisms of one sheet and the smooth surface of the other. The optical coupling prevents total internal reflection from occurring along these peaks, thus destroying the brightness enhancing effect desired. The result is a mottled and varying appearance to the backlight.
According to the present invention an optical film has structures that vary in height along their length where the variations have a nominal period of less than forty times the nominal height of the structures. In some embodiments of the invention the variation is random.